1. Field of the Invention
The present invention relates to a sheet stacking apparatus and an image forming apparatus, and more particularly, to a sheet stacking apparatus capable of aligning sheets stacked on a stacking tray, and an image forming apparatus including the sheet stacking apparatus.
2. Description of the Related Art
Conventionally, there has been known a sheet stacking apparatus capable of aligning sheets, which are discharged onto a stacking tray for stacking the sheets thereon, in a sheet discharge direction (hereinafter referred to simply as “discharge direction”) and a sheet width direction (hereinafter referred to simply as “width direction”) orthogonal to the discharge direction (see U.S. Pat. No. 7,380,786).
For example, in the sheet stacking apparatus disclosed in U.S. Pat. No. 7,380,786, when a sheet discharge portion discharges a sheet onto the stacking tray having an inclined stacking surface, a transporting portion transports the sheet toward an upstream side to bring an upstream edge of the sheet into abutment on an abutment member, thereby aligning the sheet in the discharge direction. A pair of alignment members is moved in the width direction, and the pair of alignment members is brought into abutment on both edges of the sheet in the width direction, thereby aligning the sheet in the width direction. This operation is performed every time a sheet is discharged, and the sheets are stacked while lowering the stacking tray so that the stacking height (top surface position) of the sheets stacked on the stacking tray does not exceed a predetermined height.
In the above-mentioned sheet stacking apparatus, an angle α formed between a stacking surface of a stacking tray 1137 and an abutment surface 1170 of the abutment member is acute. Therefore, when the upstream edge of a sheet abuts on the abutment surface 1170, the upstream edge may be subjected to a downward force so that the sheet is curved, resulting in a convex shape as indicated by the dotted line Sb of FIG. 19A. When sheets discharged from a sheet discharge roller pair 1130 are sequentially stacked in such a shape, an air space is formed between parts of the sheets on the abutment member side. Consequently, as illustrated in FIG. 19B, a top sheet height Ha on the abutment member side becomes relatively larger than a top sheet height Hb at the center portion in the discharge direction. Therefore, as the number of stacked sheets increases, the inclination of the sheets at a part on the downstream side of the center portion in the discharge direction is gradually reduced to approximate a horizontal posture.
A transporting portion 1301 of the above-mentioned sheet stacking apparatus moves to appear from the abutment member side into a predetermined transporting position on the stacking surface of the stacking tray 1137 to transport the sheet on the stacking tray toward the abutment member side. The stacking height of the sheets is controlled by lowering with reference to the height Ha on the abutment member side so as not to affect the discharge of the sheet from the sheet discharge portion. Therefore, when the height Ha on the abutment member side becomes relatively larger than the height Hb at the center portion in the discharge direction as the stacking amount increases, the stacking height of the sheets at the transporting position of the transporting portion may become smaller, and as a result, the pressure of contact between the transporting portion and the sheet may decrease. Accordingly, the transporting portion cannot transport the sheet so that stack misalignment may occur.
The discharged sheet is subjected to a reaction force generated when the downstream edge thereof lands on the inclined stacking surface of the stacking tray 1137, and to a sliding resistance force generated between the discharged sheet and the top surface of the sheets stacked on the stacking tray 1137 after the landing. Those forces serve as braking forces to stop the sheet after sliding by a predetermined distance. When the stacking amount of the sheets increases, the inclination of the top surface of the sheets stacked on the stacking tray 1137, which serves as a landing site of the downstream edge of the discharged sheet, is reduced to approximate a horizontal posture. Therefore, the landing reaction force and the sliding resistance force decrease and, accordingly, the sliding distance (traveling amount) increases. When the stacking amount of the sheets increases, the sheet travels away and is then stacked on the top surface of the stacked sheet having small inclination. Therefore, the sheet does not sufficiently return along the inclined surface due to the self-weight of the sheet so that stack misalignment may occur.